Physiological quality of wheat and soybean seeds produced under shade

ABSTRACT:

Solar radiation is essential in the photosynthetic process of plants, production of photoassimilates and seed formation. This study aimed to evaluate the physiological quality of wheat and soybean seeds produced under different levels of shading and stages during plant development. Field experiments were conducted with wheat and soybean crops in the 2018/18 and 2018/19 growing seasons, considered independent experiments. For both, a 4 x 3 factorial scheme was used, with four levels of artificial shading (0; 30; 50; 70% shade) and three shading periods (30-60; 60-90; 90-120 days after sowing - DAS, stages 20, 23 and 64 for wheat and V4, R2 and R5 for soybean, respectively). For wheat, germination was higher in seeds from plants shaded in the first (82%) and second periods (84%), and vigor (77% - accelerated aging - AA) was higher in seeds from plants shaded in the second period. For soybean, germination was higher (93%) in seeds from plants shaded in the third period, and vigor (AA) was lower in seeds from plants subjected to intense shading in the second period. It was concluded that, for wheat, shading from anthesis onwards favors germination and vigor, under levels of 30 to 40% imposed until anthesis; for soybean, seed germination and vigor are favored, mainly under 70% shading after the R5 stage.

Index terms:
germination; Glycine max; shading; Triticum aestivum; vigor

RESUMO:

A radiação solar é indispensável no processo fotossintético das plantas, produção dos fotoassimilados e formação das sementes. Objetivou-se avaliar a qualidade fisiológica de sementes de soja e trigo produzidas sob diferentes níveis de sombreamento e épocas do desenvolvimento. Foram conduzidos experimentos de campo, com as culturas do trigo e soja, na safra 2017/2018, considerados experimentos independentes. Para ambos se utilizou esquema fatorial 4 x 3, sendo: quatro níveis de sombreamento artificial (0; 30; 50; 70% de sombra); e três épocas de sombreamento (30-60; 60-90; 90-120 dias após a semeadura -DAS, estágios 20; 23 e 64 para trigo e, V4; R2 e R5 para soja, respectivamente). Para trigo, a germinação foi maior para sementes de plantas sombreadas na 1ª (82%) e 2ª época (84%), e maior vigor (77% - envelhecimento acelerado- EA) sob sombreamento na 2ª época. Para soja, a maior germinação (93%) foi para a terceira época, e o vigor (EA) foi menor sob intenso sombreamento na 2ª época. Concluiu-se que, para trigo, sombreamento, na média das intensidades a partir da antese favorece a germinação, e o vigor sob níveis de 30 a 40% impostos até a antese; para soja, a germinação e vigor de sementes são favorecidos, principalmente sob 70% de sombra a partir do estágio R5.

Termos para indexação:
germinação; Glicyne max; sombreamento; Triticum aestivum; vigor

INTRODUCTION

The success in the initial establishment of seedlings and the vigor of seeds is determined by physiological and biochemical traits and are related to the reserves available for seedling development until the autotrophic stage (Marcos-Filho, 2015). The reduction of photoassimilates interferes with the source-sink relationship of plants, affecting the number of seeds, weight of seeds, yield, flowering efficiency, and dry matter translocation (Marcos-Filho, 2015; Asseng et al., 2017).

The limitation of carbon in the source tissues and consequent conversion of sugars into starch in the sink tissues (seeds) can occur when plants are shaded, interfering with the development of their progeny (Vayda et al., 2018). Some of the situations that limit solar radiation on plants are integrated crop-livestock-forest systems (ICLFS) (Cordeiro et al., 2015) and high cloudiness (Custódio et al., 2009). Responses to sunlight limitation included reduction in protein and lipid levels and variation in seed germination in soybean plants (50% shading) (Nacer et al., 2011), higher root protrusion speed and seedling performance in soybean (51 and 38% shading) (Chen et al., 2020), high germination of soybean seeds (87%) (Cigel et al., 2023) and wheat seeds (98%) (Cigel et al., 2021), and positive effect of shading (35% intensity) in the final stages of seed development on the germination and establishment of sunflower seedlings (Huang et al., 2024).

The source-sink relationship is fundamental in seed formation, as it directly defines the genetic potential, quality, and production performance of crops such as soybean and wheat, especially under adverse environmental conditions (Fioreze et al., 2021; Fioreze et al., 2022). In the seed formation phase of soybean and wheat, the source-sink relationship plays a fundamental role in defining the quality of the seeds and their production performance. Leaves and other plant tissues act as sources of assimilates, while seeds in formation are sinks, receiving these nutrients. During this period, the balance between the production of photoassimilates by the sources and the growing demand of sinks is essential to ensure the efficient filling of seeds with carbohydrates, proteins and lipids. Thus, environmental stress that affects this relationship can result in lower quality seeds, which will also be associated with the low production potential of these crops (Fioreze et al., 2021; Fioreze et al., 2022).

Therefore, it is relevant to conduct studies to improve the understanding of the interference of shading in the most critical stages of crop development and its consequence on seed quality, such as in ICLF or ICF systems. The objective of this study was to evaluate the influence of shading levels imposed at different stages of plant development on the physiological quality of the seeds produced by wheat and soybean crops.

MATERIAL AND METHODS

The experiments were carried out at the Center for Agricultural Sciences of the Universidade Estadual de Santa Catarina (Udesc), in the municipality of Lages, SC, Brazil, at the geographic coordinates 27°52’ South latitude and 50°18’ West Longitude, at an average altitude of 930 m. Seeds were produced under field conditions, in the experimental area of Udesc, in a wheat/soybean succession system, with the wheat cultivar Esporão (2018/2018) and the soybean cultivar 96Y90 (2018/2019). The records of temperature, incident solar radiation (unshaded) and precipitation (Figure 1) during the experiments were obtained from the National Institute of Meteorology (INMET, 2019).

Figure 1
Daily precipitation (Precip), minimum temperature (min T°), mean temperature (mean T°) and maximum temperature (max T°) (a), and global solar radiation (GSR) (b) recorded during the experiments with wheat (a, b) and soybean (c, d) crops. Source: INMET, 2019.

Shading was created using metallic structures, with dimensions of 1.2 m x 1.2 m x 1.2 m (width, length and height), wrapped with wire and black polyethylene meshes of different fiber densities, 30, 50 and 70%, verified and proven via Photosynthetically Active Radiation (PAR) and Photosynthetic Photonic Flux Density (PPFD) measurements (Light Sensor Logger LI-1500, LICOR®, Lincoln, NE, USA). For the control treatments, the structures had only the wire mesh (to prevent herbivory by birds).

The experimental design used in the field was completely randomized blocks, in a 3 x 4 factorial scheme, with three shading periods (starting at 30, 60, and 90 days after sowing - DAS) and four shading levels (0, 30, 50 and 70%). The shading periods consisted of intervals of 30 days. Four replications were used for each treatment, each corresponding to one experimental plot.

The shading periods in wheat started on: August 10 - phenological stage 20 (1^st period); September 10 - phenological stage 23 (2^nd period); October 10, 2018 - phenological stage 65 (3^rd period) according to the scale of Zadoks et al. (1974). The plots were composed of five sowing rows, 1.20 m long and spaced 0.2 m apart. For soybean crop, the shading periods started on: January 28 - phenological stage V4 (1^st period); February 28 - phenological stage R2 (2^nd period); and March 28 - phenological stage R5 (3^rd period), according to the scale of Fehr and Caviness (1971). Each plot was composed of three sowing rows, 1.2 m long and spaced 0.4 m apart.

After mechanical harvesting and pre-drying, the seeds of the four plots of each treatment were joined to form a lot, homogenized to form the average sample, and then separated into four replications to form the work samples. Analyses were carried out using seeds retained on a sieve with a transverse diameter of 1.75 mm for wheat and seeds retained on a sieve with a diameter of 3.5 mm and circular openings for soybean. Subsequently, these seeds were subjected to evaluations of Thousand-Seed Weight (TSW), moisture percentage (MO%) by the oven method at 105 °C ± 3 °C, for 24 hours, and Germination test (G) with 400 seeds, according to the Rules for Seed Testing (Brasil, 2009).

Seed vigor was evaluated based on the Germination Speed Index (GSI) for wheat (between paper) and Emergence Speed Index (ESI) for soybean (between sand) (Maguire, 1962); Accelerated Aging (AA) at 42 °C for 60 hours for wheat and at 41 °C for 48 hours for soybean (Marcos-Filho, 2020); biochemical test of Electrical Conductivity (EC) with benchtop conductivity meter (Quimis, Model 0795A2) (Krzyzanowski et al., 2020); and based on seedling performance, verified by total seedling length (TSL), obtained by measuring 20 seedlings on the eighth day of the germination test, and dry mass of seedlings (DMS), measured after drying in a forced air circulation oven at 65 °C until weight stabilization (Krzyzanowski et al., 2020) and expressed in mg per seedling.

The data were subjected to normality and homogeneity tests, and the results were analyzed by the F test at 5% significance level. Data that did not meet the assumptions were transformed into percentages through arcsine of [x/100]^0.5 or arcsine of [(x+1.5)/100]^0.5. The standard error of the means was calculated by SD/(n^0.5) where SD: standard deviation, and n: number of replications. When significant, means of qualitative variables (period) were compared using the Skott-Knott test (p≤ 0.05 probability of error) and for quantitative variables (shading), regression analysis was performed. Principal component analysis (PCA) was performed with the means of the shading levels and periods for each trait evaluated. All analyses were carried out using the statistical software R (R Core Team, 2025).

RESULTS

The first result that must be presented in this type of study is the seed yield in kilograms per hectare of each treatment so that the feasibility of the technique can be evaluated. Even if seed quality is excellent in a treatment, this treatment does not make sense if the yield is very low, insignificant. Thousand-seed weight is important, but it does not replace yield.

Analyses of seeds from plants subjected to shading levels at different stages of development did not show a significant effect on total seedling length for wheat (TSL) and vigor by the electrical conductivity (EC) test for soybean.

The TSW of wheat and soybean subjected to shading levels at different stages of development showed an effect of interaction between the factors. For wheat shaded during the 1^st period, the highest TSW (39.2 g) was obtained under shading of 31.8%, and in the 2^nd period, with 39.2 g, under 28.7%. For light restriction imposed in the 3^rd period, losses were found in the TSW of wheat (Figure 2a). The TSW of soybean for seeds produced under light restriction in the 1^st period was higher at the shading level of 46.7% (138.5 g), but in the 2^nd period there was an increase in TSW under shading above 5.2%. In the 3^rd period, a decreasing linear behavior was observed, with a reduction of 2.3 g in TSW for each 10% increase in the shading level imposed on the crop (Figure 2b).

Figure 2
Thousand-seed weight (TSW) (a; b), moisture percentage (MO%) (c; d) and germination (e; f) of wheat and soybean seeds as a function of shading levels on plants at different stages of development. Bars represent the standard error of the mean. Equal means do not differ from each other by the Skott-Knott test (p ≤ 0.05).

The moisture percentage (MO%) of wheat seeds was significantly affected only by the shading factor, with quadratic behavior, in which the highest percentage, 11.2%, was observed under light restriction of 60% (Figure 2c). In relation to soybean seeds, for shading in the 2^nd period, under intensity above 15%, there was an increase in moisture percentage as the intensity increased, while at the 70% level, moisture percentage was higher than in the other periods under the same condition (Figure 2d).

For wheat germination (G), there was no significant effect of shading intensity, but seeds produced under shading in the 2^nd and 3^rd periods showed the highest percentages, 82 and 84%, respectively (Figure 2e). For soybean, light restriction during the 3^rd period had an effect on the G of the seeds, which had the highest percentage, 93%, under the shading level of 53.6%; however, regardless of shading intensity, the means were above 85% (Figure 2f).

For the vigor by the GSI and ESI tests, there was an effect only of the shading periods; wheat and soybean seeds showed significance for the 3^rd period (anthesis to harvest maturity). For wheat, there was an increase of 7.8% compared to the 1^st and 2^nd periods, whose GSI was 18.3 and 18.9, respectively (Figure 3a), and for the ESI of soybean, there was an increase of 3.63% compared to the 2^nd period (Figure 3b).

Figure 3
Germination and emergence speed indices (GSI, ESI) (a; b), vigor by accelerated aging test (AA) (c; d), electrical conductivity (EC) (e), total seedling length (TSL) (f) and dry mass of seedlings (DMS) (g; h) of wheat and soybean as a function of shading levels on plants at different stages of development. Bars represent the standard error of the mean. Equal means do not differ from each other by the Skott-Knott test (p ≤ 0.05).

The vigor of wheat seeds by the accelerated aging (AA) test, under shading in the 1^st and 2^nd periods, was higher (78 and 77%) under shading levels of 30.6 and 41.2%, respectively. However, for the 3^rd period, the lowest seed vigor (58%) was obtained under shading of 32%, with an increase under higher levels, but which did not differ from the control (Figure 3c). The vigor of soybean by AA under shading in the 1^st and 3^rd period showed quadratic behavior with minimum vigor, reduction up to percentages of 61 and 63%, under shading intensity of 32.0 and 28.6%, respectively; however, under higher intensity there was an increase in vigor by AA. For soybean seeds produced under shading during the 2^nd period, there was a linear reduction (3%) in vigor for each 10% increase in the shading level (Figure 3d).

The vigor of wheat seeds by EC was significantly affected by the shading level factor, highlighting its negative effects on the vigor by the test. An increasing linear behavior was observed, with increment of 0.3 μS.g^-1.cm^-1 in the EC of the seeds for each 10% increase in shading intensity (Figure 3e), with a consequent reduction in vigor.

The dry mass of wheat seedlings (DMS) showed a quadratic behavior for the 1^st period, with the highest mean (50.2 mg per seedling) under a shading level of 32.1%. For the restriction in the 2^nd and 3^rd period, there were reductions of 0.7 and 2.2 mg per seedling for each 10% increase in shading, respectively. For the 3^rd period, seedlings with lower DMS, 20.7 mg per seedling, were observed at shading level of 70% (Figure 3g).

The TSL of soybean seeds from the 1^st period showed a linear behavior with an increase of 0.5 cm for each 10% increase in shading level. However, for those from the 2^nd period, there was a reduction of 0.3 cm for each 10% increase in light restriction (Figure 3f). For the DMS of soybean, values between 95.0 and 106.1 mg were observed for the 1st shading period. However, for the 2^nd period there was a quadratic behavior, with a reduction of up to 90.3 mg, under shading of 17.9%. For the 3^rd period, there was a reduction in DMS of 1.7 mg for each 10% increase in the intensity of the limitation imposed (Figure 3h).

The analysis of the principal component matrix (PCA) for seed quality, at the shading intensities and periods evaluated, components 1 and 2, represented by the x and y axes, explained 83.6% of the variations for wheat (Figure 4a) and only 74.9% for soybean (Figure 4b). When analyzing the intensity that was positive for the quality parameters with the control treatment, shading of 30% and the control (0%), the PCA explained, among the first two components, for wheat, the variations in 96.0% (Figure 4c), and the variables with the greatest contribution were GSI, AA and DMS. Association with shading in the 3^rd period was observed for G, GSI and TSL, but there were reductions in vigor by AA, DMS and TSW (Figure 4c).

Figure 4
Principal component analysis (PCA) matrix for the quality parameters for wheat and soybean seeds as a function of shading intensities of 0, 30, 50 and 70% (a, b), and 0 and 30% (c, d), at different periods of plant development. *Numbers 0, 30, 50 and 70 refer to shading intensities: 0, 30, 50 and 70%, respectively; Numbers 1, 2 and 3 refer to the periods of development of crops; G: Germination; ESI: Emergence speed index; GSI: Germination speed index; EC: electrical conductivity; CT: cold test; AA: accelerated aging; TSL: total seedling length; DMS: dry mass of seedlings; TWS: thousand-seed weight.

For soybean seed quality, shading of 30% and control (0%) explained 85.9% of the variations (Figure 4d), and the variables EC, AA, TSW, TSL, G and DMS had the greatest contribution. For this crop, there were positive and direct associations of DMS and TWS with shading of 30% in the 1^st and 2^nd periods, and negative associations between G, ESI, CT and TSL and shading in the 3^rd period of crop development (Figure 4d).

DISCUSSION

Considering the source-sink relationship, the yield can be limited by the reduction of the source during the seed filling phase; however, it is also influenced by the size of the sink, whose moment of definition is during flowering (Zhang and Flottmann, 2018). The source-sink relationship is of indispensable importance in the seed formation phase, as leaves and plant tissues provide assimilates, and developing seeds absorb them (Fioreze et al., 2021; Fioreze et al., 2022). Differences were observed regarding the ability to use stored photoassimilates during seed filling. During the study, the period that comprises the filling stage of soybean seeds has a lower amount of solar radiation available compared to the period of definition of the number of seeds; however, the relationship is inverse with the wheat crop, due to the seasons and length of the day, corroborating the results (Figure 1).

High shading intensities at the beginning of wheat development may have affected biomass accumulation and restricted the later stages of development, with less remobilization of photoassimilates (Asseng et al., 2017), as seed filling is mainly dependent on the photosynthesis performed by the flag leaf and the high remobilization rate (Li et al., 2010). Wheat sensitivity to shading during anthesis and seed filling in terms of TSW may be due to competition for photoassimilates, as the remobilization and photosynthetic rate in post-anthesis may not have met the energy demand of the seeds, even with a delay in leaf senescence (Li et al., 2014; Xu et al., 2016; Araki et al., 2025). The results indicated a possible compensation for the lower number of seeds (data not shown) with a higher weight, corroborating Labra et al. (2017) and Asseng et al. (2017).

For soybean, the results of TSW with shading in the 1^st period may be due to the etiolation of the plants under lower radiation intensity (Yang et al., 2018), resulting from the increase in internode length and lower branching (Liu et al., 2015). For the reproductive phase (2^nd and 3^rd periods) a compensation behavior was observed, similar to that found in wheat, which may be related to the limitation of photoassimilates during flowering (Zhang and Flottmann, 2018), and, in the 3^rd period, may have reduced the seed filling rate, since wheat, soybean and maize, according to Borrás et al. (2004), are dependent on the amount of photoassimilates produced during this period, which differed from the results obtained in wheat, but it was similar with soybean.

The higher moisture percentage of wheat and soybean seeds under a high level of shading can be justified by the delay in development and consequently in harvest maturity, corroborating Xu et al. (2016) and Yasin et al. (2019). For soybean, the higher moisture percentage and delay in plant senescence by shading may be related to the occurrence of green stem disorder, as observed in related studies; however, further investigation is needed (Yamazaki et al., 2018).

Wheat seed germination was favored by the low shading intensity in the final stages of development, confirmed by PCA (Figure 4e). This may be due to the lower temperature in shaded environments (Li et al., 2010), as observed for the germination of soybean shaded in the third period, corroborating Nacer et al. (2011), or to the possible increase in lignin content in the seed coat and consequently in the resistance of seeds to deterioration (not evaluated), already observed in seeds produced under shade stress during the reproductive stage (Deng et al., 2025). For both crops, the higher germination speed coincided with seeds with lower TSW (Figure 2a and 2b), as smaller seeds require less water for imbibition (Marcos-Filho, 2015).

For the vigor of wheat seeds by AA, it is possible to infer a beneficial effect of light shading at the beginning of development, avoiding photosynthetic saturation or increasing radiation use efficiency (Kanniah et al., 2012). However, under high shading, there may have been a limitation in the subsequent readaptation to stress, with less accumulation of photoassimilates in the vegetative parts, lower photosynthetic rate, culminating in less remobilization, whose contribution is important for the formation of the endosperm (Li et al., 2010). The vigor of wheat seeds from 32.0% light restriction in the 3^rd period may be related to their nitrogen concentration (data not evaluated), as this is the main source for the seeds prior to anthesis (Kong et al., 2016).

The vigor by AA of soybean, shaded in the 1^st period, can be explained by the lower number of seeds having determined their higher weight, because, according to Yamazaki et al (2018), the end of light restriction and then return of the incidence of solar radiation from the R1 stage makes it possible to increase the number of pods per plant, favoring yield. As for shading in the 2^nd period, there were losses of vigor possibly due to the delay in plant senescence and nitrogen translocation to the seeds (Yamazaki et al., 2018), as the lower nitrogen remobilization can be considered the main responsible for the reduction in the protein concentration of seeds (Proulx and Naeve, 2009), lower vigor (Henning et al., 2010) and efficiency in the remobilization of reserves (Andrade et al., 2019).

The reduction of vigor in wheat seeds detected under intense shading may not have been sufficient to affect vigor through other evaluation methods, as the EC test assesses seed vigor based on the ability of membranes to reorganize after imbibition, corroborating whit Favoretto et al. (2024). That is, low-vigor seeds have a lower membrane organization capacity and a greater release of ions and solutes (Krzyzanowski et al., 2020).

High-vigor seeds have high levels of proteins and soluble sugars, starch, in addition to a greater capacity to mobilize reserves during germination, producing seedlings with better performance (Henning et al., 2010; Padilha et al., 2022). It can be stated that the higher vigor of wheat seedlings produced under shading in the 1^st period was related to the beneficial effect of light shading on photosynthesis and production of photoassimilates, destined to seeds, as already discussed, in addition to the high TSW. For soybean, larger seeds originated more vigorous seedlings, which is related to their greater capacity to translocate and incorporate the accumulated reserves to the embryonic axis (Padilha et al., 2020).

CONCLUSIONS

Shading of wheat plants from anthesis (second period) produces seeds with higher germination; however, vigor is favored under shading levels of 30 to 40% imposed until anthesis (first and second period of development). For soybean, shading on mother plants at the end of the cycle, from the R5 stage, promoted greater germination and vigor of the seeds produced, but it is highly harmful to vigor when imposed at the R2 stage.

ACKNOWLEDGMENTS

The authors would like to thank CNPq, CAPES, FAPESC/UDESC/PAP and UNIEDU/FUMDES for their financial support to the present research and the scholarships granted to the authors.

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• DATA AVAILABILITY
  Additional data will be made available by the authors upon reasonable request.

Edited by

• Editor:
  Hugo César Rodrigues Moreira Catão

Data availability

Additional data will be made available by the authors upon reasonable request.

Publication Dates

• Publication in this collection
  14 Nov 2025
• Date of issue
  2025

History

• Received
  04 Nov 2024
• Accepted
  22 Sept 2025